Recently, natural gas has came to attract attention as a substitute energy source for gasoline because of increasing oil prices and mounting environmental problems. Accordingly, using natural gas for the fuel of the internal combustion engines for motor vehicles is eagerly proposed. Natural gas is normally stored in a bottle mounted in a vehicle as compressed natural gas (hereinafter referred to as CNG). However, CNG is not readily available because the number of CNG gas stations is so limited. Therefore, a dual-fuel internal combustion engine which can selectively use either gasoline or CNG is proposed. According to the structure of the dual-fuel engine disclosed in Japanese patent laid open publication No. 62-214238, gasoline and CNG can be selectively used depending on the driving condition.
In conventional dual-fuel engines, a common intake passage and a common intake valve for both of the fuels are provided for each cylinder. And a gasoline injection system and a CNG injection system are provided in the common intake passage. The desirable combustion timing, and the desirable actuating timing and appropriate lift of the intake valves are different for each fuel. However, in reality, either the combustion condition is fixed to that for gasoline or some compromise is made by using a same set of conditions for gasoline and CNG.
In either case, when the fuel is switched from gasoline to CNG, the mixture of CNG and gasoline is supplied into the combustion chamber for a while because gasoline inevitably adheres around the intake valves. As a result, the combustion conditions sometimes deviate so much from the initially designed ones that emission properties are impaired, and the output torque is reduced.